In the package of semiconductor chips, the use of CSP (Chip Scale Package) has proceeded and a mold-releasing film is used in place of compounding a mold-releasing agent with a encapsulating resin to increase the mold-releasing property, whereby the improvement of the working efficiency by the reduction of a mold-cleaning frequency, the reduction of cost by prolonging the life of mold, etc., have been attempted. In this case, it is proposed to use a polytetrafluoroethylene film as the releasing film (JP-A-8-197567, JP-A-8-186141, etc.) (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
FIG. 1 shows QFN (Quad Flat Package) of the structure formed by removing the lower surface mold portion from QFP and cutting off the lead at the side from the mold side portion. In the case of carrying out resin encapsulating thereof, the electrode of a semiconductor chip 2 is bonded to the terminal 22 of each lead frame 21 with a wire, the semiconductor chip 2 is placed in the cavity 11 of a lower mold 1 with the terminals 22 being at the upper side as shown in FIG. 1(A), the mold is closed by an upper mold 3 via a polytetrafluoroethylene film F and a resin is injected into the cavity by a transfer molding and is simultaneously cured as shown in FIG. 1(B), the mold is then opened as shown in FIG. 1(C), and the lead frames are trimmed leaving the terminals.
In the resin-encapsulating method, it is expected that the terminals bites into the polytetrafluoroethylene film by the mold closing pressure, whereby the terminals and the polytetrafluoroethylene film are always maintained at a contact state during resin encapsulating to exclude the occurrence of resin covering of the terminals.
As well known, the polytetrafluoroethylene film can be obtained by a casting method (a method of coating a dispersion of polytetrafluoroethylene on a carrier sheet followed by sintering and releasing the sintered film from the carrier sheet), a cutting method (a method of sintering a polytetrafluoroethylene rod and cutting the rod by a cutting lathe into a film), etc.
However, when the present inventors performed resin-encapsulating of semiconductor chips using the polytetrafluoroethylene film as the mold-releasing film, in spite of that there was no substantial difference in the modulus of elasticity and the exclusion of the occurrence of resin-covering of the terminal was similarly expected by cutting the terminals into the polytetrafluoroethylene film by mold-closing pressure to always maintain the terminals and the polytetrafluoroethylene film at a contact state during resin encapsulating, it has been found that the generation ratio of resin covering (inferiority ratio) of the above-described terminals considerably differs (practically, there is a difference of the inferiority ratio in the range of from 0 to 10/50) according to the difference in the above-described production method of the polytetrafluoroethylene film.
Accordingly, the inventors made the investigations on the cause thereof and have found that the cause is in the difference in the thermal shrinkage percentages between the lengthwise direction and the width direction of the mold-releasing film. That is, it is presumed that when a film has a directional property in the thermal shrinkage percentage, a force which shears and deforms the film slantingly is applied to the film to thereby cause creases.